1. Field of the Invention
The present invention relates to a droop-free quasi-continuous reconstruction filter interface. More specifically, a reconstruction filter interface is disclosed that resamples the output of a digital-to-analog converter (DAC) and eliminates problems in the output of the DAC such as slewing, glitching, and droop.
2. Description of the Related Art
Asymmetric Digital Subscriber Line (ADSL) technology is used to deliver high-rate digital data over existing phone lines without modification of the phone lines. A modulation scheme called Discrete Multitone (DMT) allows the transmission of high speed digital data. ADSL allows a large bandwidth (1.104 MHz) for transmission in a downstream direction from a central office to a subscriber in a remote location and a lesser bandwidth (138 kHz) for transmission in an upstream direction from of a subscriber in a remote location to a central office. ADSL systems are described further in the ADSL standards ITU-g.992.1 and ITU-g.992.2, which are herein incorporated by reference for all purposes.
FIG. 1A illustrates the architecture of an ADSL system. A digital network 100 provides data that is to be sent downstream over the ADSL line. The data is delivered to an ADSL transceiver unit in a central office (an ATU-C) 102. The ATU-C modulates data for downstream transmission using a DMT scheme. ATU-C drives a line 104 with an analog signal. Line 104 is connected to a telephone line 110 that is part of a Public Switched Telephone Network (PSTN) 106 by a splitter 108. ATU-C also receives data transmitted in the upstream direction on line 104.
At the receiver end, a splitter 120 splits the signal on line 110 between a message telecommunication service (MTS) 122 (also referred to as plain old telephone service (POTS)) and an ADSL transceiver unit line for a remote unit (an ATU-R) 124. ATU-R 124 is connected to one or more service modules (SM) 128 by a bus 126. The ATU-R demodulates data transmitted by the ATU-C in the downstream direction and also transmits data in the upsteam direction.
FIG. 1B is a block diagram illustrates the transmission path of an ADSL system. The transmission path may be in either the upstream or the downstream direction. An encoder 150 sends digital data to a DAC 152. The analog output of DAC 152 represent an ADSL modulated signal that is to be transmitted on an ADSL line. However, the DAC is not a suitable line driver because it generally does not have sufficient power output and also because the output of the DAC contains a large amount of spurious, out of band harmonics generated by the zero order hold nature of a realizable analog DAC. A line driver amplifier 153 is used to increase the power drive capability and a reconstruction filter 154 reduces or eliminates the out of band harmonics. The output of the DAC is directed to reconstruction filter 154 and the output of reconstruction filter 154 is directed to line drive amplifier 153. Reconstruction filter 154 reconstructs the signal from the stairstep output of the DAC for the purpose of driving an ADSL line 156. Thus, the signal that drives ADSL line 156 is first generated digitally by encoder 150. DAC 152 converts the digital signal to an analog signal and reconstruction filter 154 drives the line driver, which in turn directs the signal into the line proper.
The performance of reconstruction filter 154 is important because, without reconstruction filter 154, the output of the ADSL transmitter would include residual harmonic energy over a broad spectrum and would likely violate FCC rules as well as have a poorly defined in band spectrum shape. Furthermore, any high frequency glitching and slew energy would be present on the line, further distorting and corrupting the signal.
FIG. 2 is a block diagram illustrating the basic structure of a typical reconstruction filter. The output of a DAC 200 is input to an amplifier 201 configured in a low pass circuit that includes an input resistor 202, a feedback resistor 204 and a capacitor 206. In this low pass configuration, the output of the amplifier at node 210 tends to follow only the low frequency content of the output of the DAC within the design limitations of amplifier 201, thus achieving reconstruction of the signal.
When the output of the digital-to-analog converter is interfaced directly into the associated reconstruction filter in a continuous time fashion as shown above, several problems arc caused due to the fact that the DAC is inherently discrete time in nature. First, any form of nonlinear settling transient in the DAC such as glitching or slewing distortion is reproduced by the construction filter in the line output. Glitching 320 and slewing 310 are illustrated in FIG. 3B. Also, the fact that the DAC output is a zero order held waveform results in a frequency dependent droop across the output band, which makes necessary some form of X/sin(X) correction either in the digital processing prior to the DAC or in the analog filter itself. Typically, such correction is costly, both in terms of speed and complexity.
It would be desirable if a system could be designed that could eliminate the problems caused by slewing, glitching and droop that result from applying the output of the digital-to-analog converter to the reconstruction filter.